1. Field of the Invention
The present invention generally relates to electrical circuits for supplying energy to multiple loads, such as CCFL (Cold-Cathode-Fluorescent-Lamp) loads and more particularly to electrical circuits for providing phase shifts to multiple loads. Usually, the electrical circuits are applied to display devices, such as liquid crystal display monitors, liquid crystal display computers or liquid crystal display televisions.
2. Description of the Related Art
CCFL loads are extensively used to provide backlighting for liquid crystal displays (LCD), particularly for backlighting LCD monitors and LCD televisions. However, such conventional applications require a separate direct current/alternative current power inverter (DC/AC power inverter) to drive an individual CCFL. Such an application is illustrated in FIG. 1, wherein each CCFL (20, 22, . . . 24) is powered by an individual DC/AC inverter (10, 12, . . . 14), respectively, and all DC/AC inverters are synchronized. Each DC/AC inverter includes a switched AC network and a power driver circuit. The power driver circuit may include a resonant tank circuit for the CCFL. The switched AC network in each inverter is driven ON/OFF synchronously. Therefore, there is a large ripple on the power line. A large current will be drawn from the power source VBatt when the switches in the switched networks are turned on and the current drawn is released when the switches are turned off. The simultaneous turning on and off at all inverters cause noises on the power line which degrades the signal/noise integrity in the system.
One method to reduce the ripple is to increase the filtering at the power line. However, the disadvantage is that the size of the circuit is increased, which in turn increases the system cost.
FIG. 2 shows another prior art circuit for driving multiple CCFL loads where the circuit includes a controller (40) for driving multiple DC/AC inverters (10, 12, 14, . . . 16) and CCFL loads (20, 22, 24, . . . 26). A clock generator (42) in the controller (40) generates a string of phase-shifted clock signals to each DC/AC inverter (10, 12, 14, . . . 16) to make a phase delay. Since the switches in the networks of all DC/AC inverters (10, 12, 14, . . . 16) are turned on and turned off with equal phase shift between the adjacent inverters, the ripple on the power line is effectively reduced to 1/N of that shown in FIG. 1, where N is the number of DC/AC inverters connected.
However, the problem is that the controller (40) is fixed to the number of original demanded loads, in other words, the number of CCFL loads equals to the lines which deliver phase shifts from the controller (40) to each inverter (10, 12, 14, . . . 16). Therefore, if the number of CCFL loads is changed, the configuration of the controller (40) should be changed. Another disadvantage is that the controller (40) needs to generate a high-frequency clock signal having a frequency of N times the operating frequency of the individual DC/AC inverter.